Kerbal Space Program

When I am looking at the Kerbal Space Center overview and I click on the launchpad, the menu comes up as usual, with a list of crafts and available astronauts and a mission flag. I am able to select a craft from the list; its description will appear, and the astronauts lists will populate correctly, but when I click the green "Launch" button on the top-right corner of the menu, it simply closes the box and I am returned to the KSC overview.

I have tried this process with various crafts, both stock and my own creations.
I have tried launching spaceplanes from the runway, this works fine every time.
I have tried different crew arrangements.
I have tried various mission flags.